2012-08-31_REVISION - M1980244 - Laserfiche WebLink

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2. There is a strong downward vertical hydraulic gradient in areas of the Precambrian rocks close to the Diatreme where the slope of the interface is steep (e.g. in upper Grassy Valley). 3. There is weak to no downward vertical hydraulic gradient in locations underlain by Precambrian rocks that are distant from the Diatreme, or are adjacent to locations where the interface between the slope of the interface is shallow (e.g. in lower Grassy Valley). In all areas where there is moderate to strong downward vertical gradient, ground water flow is predominantly vertically downward, and all infiltrating water is captured by the diatremal flow system and flows via the regional ground water system to Carlton Tunnel. DRMS Response Nos. 41c, 47c & d — The Division reserves the right to comment on these responses when the final responses from AMEC are submitted. CC& V Reply: CC& V provides a reply from AMEC herein to Response Nos. 41c, 47c & d The VLF design also includes a closure underdrain. The purpose of the closure underdrain is to provide a means to drain accumulated precipitation water from the VLF at and during the post closure period (after the ore heap has been rinsed), to prevent overtopping of the PSSA embankment. The closure underdrain consists of the following components: • A blanket drain that is located beneath the PSSA sump (i.e., low point); • A thicker (i.e., 3-foot) layer of SLF underlying the lower geomembrane within the portion of the PSSA which overlies the closure drain; and • A passive drain consisting of five drilled shafts extending a minimum of 10 feet into the diatreme zone. During the post closure period the blanket drain beneath the PSSA will be penetrated and used to convey accumulated water within the PSSA to the primary sump. The blanket drain consists of a minimum 3-foot thick layer of drainage gravel (underdrain fill) enclosed in a geotextile. During the closure period, after the PSSA water level has been reduced to a maximum level of 8 feet, the PSSA liner system will be punctured by drilling a borehole through the PSSA and into the blanket drain. This will facilitate long -term drainage of the PSSA. The second component of the closure underdrain is passive drainage of water from the VLF into the Diatreme beneath the facility. During construction of the closure drain during Phase 1 construction of the SGVLF, drilled shafts will be advanced a minimum of 10 feet into the Diatreme foundation. Adrian Brown (Cresson Project Hydrogeochemical Evaluation, Attachment 1, Volume II of MLE2 Application) presents geologic maps for the area (see Figure 1 in Attachment 3). According to this map, the closure drains will be drilled through Precambrian aged Cripple Creek Quartz Monzonite prior to reaching the Tertiary aged Cripple Creek Lapilli Breccia (Diatreme). According to this report, the saturated hydraulic conductivity of the Diatreme has a geometric mean of 2 x 10 cm/sec; and the saturated hydraulic conductivity of the native rock has a geometric mean of 2 x 10 cm/sec. As shown, the Diatreme is two orders of magnitude more permeable than the surrounding rock. In addition, it is important to note that the hydraulic conductivity of the Cripple Creek Quartz Monzonite is approximately equal to the hydraulic conductivity of the soil liner for the VLF. Given that the Diatreme is significantly more permeable than the low permeability of the Cripple Creek Quartz Monzonite , the slope of the closure drains just needs to be sufficient to maintain free- drainage. From a practical standpoint, drilling the holes at approximately 15 degrees from horizontal would be considered the minimum. DRMS Response Attachment 10, Mill Site Earthworks Drawings, dated August 16, 2012: A170 — Please confirm the 48 -inch diameter HDPE SDR11 riser pipe is adequate to support itself given the number of perforations at the bottom.